Surface electronic bands of submonolayer Ge on Ag(111)

Germanium deposited on single-crystal Ag(111) substrates at about 1/3-monolayer coverage forms a stable $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ superstructure as seen by reflection high-energy electron diffraction. In situ angle resolved photoelectron spectroscopy reveals a rich band structure consisting of conelike features that cross the Fermi surface at the center ($\overline{\ensuremath{\Gamma}}$ points) of the superstructure surface Brillouin zone (SBZ), exhibiting a high dispersion with the surface parallel wave vector. With the help of first-principles calculations, it can be inferred that most of the features are explained in terms of an Ag${}_{2}$Ge-ordered surface alloy corresponding to surface states localized in the first two layers. One of the bands has a saddle point near the $\overline{\mathrm{M}}$ point of the SBZ with a characteristic split-band behavior distinguishing Ge surface alloy from other similar systems.